Such an electroimpedance tomograph (EIT) is known, for example, from EP 1 000 580 A1, which is used to record an “electroimpedance tomographic image” of a body section of a patient. A corresponding electroimpedance tomography method performed at different frequencies is known from EP 0 669 822 A1.
Electric impedance tomography is a method for the reconstruction of impedance distributions, more precisely, of impedance changes relative to a reference distribution, in electrically conductive bodies. A plurality of electrodes are arranged for this purpose on the conductive surface of the body to be examined, and the control unit, usually a digital signal processor, ensures that a pair of (preferably) adjacent electrodes wherein each is supplied with an alternating electric current (for example, 5 mA at 50 kHz), and the electric voltages are detected at the remaining electrodes and are sent to the control unit. Due to the combination of the measured voltage values during the consecutive rotating current feed, the impedance distribution, more precisely, the change in this distribution compared with a reference distribution, can be reconstructed with suitable algorithms. A ring-shaped equidistant array of 16 electrodes, which can be placed around the body of a patient, for example, with a belt, is used in typical cases. Alternating current is fed into two adjacent electrodes each, and the voltages are measured between the remaining currentless electrode pairs and recorded by the control unit. Due to the rotation of the current feed points, a plurality of measured voltage values are obtained, from which a two-dimensional tomogram of the impedance distribution relative to a reference can be reconstructed in the electrode plane.
Such tomograms are of interest in medicine, because the impedances depend on the biological state of the organs (for example, the state of respiration of the lungs) and/or on the frequency of the current. Both measurements in different states with a given feed frequency and different biological states (for example, observation of the breathing cycles) and measurements performed at different frequencies with different feed frequencies and equal biological state are therefore performed in order to obtain information on the corresponding impedance changes. As was mentioned above, the functional impedance tomography of the lung, in which the electrodes of the EIT are placed around the chest of the patient, is an important application.
An EIT typically comprises a number of electrodes, which can be placed, in particular, on a carrier around the body to be examined in a ring-shaped pattern, and analog electronic circuits for the signal amplification and for the alternating current feed, and digital electronic circuits for digitizing and preprocessing the voltage signals as well as for controlling the current feed, a digital connection with a control unit for controlling the apparatus and for processing the recorded data for the reconstruction of the impedances, as well as a monitor for displaying the impedance distribution. The term “control unit” is used here in a broad sense of this word and it designates a processor unit that both controls the operation of the EIT and performs the evaluation of the detected signals for the reconstruction of the impedance distribution as well as additional analysis operations. A visualization of the reconstructed impedance distribution is then displayed on a monitor.
In prior-art EITs, the frequency (or the frequencies in case of multifrequency measurement) of the feed current is set at the beginning of the measurement manually or by a fixed, stored value and left unchanged, or there is a possibility of changing the frequency of the feed current manually at a later point in time. This procedure is sufficient for laboratory applications, but is unsuitable for the routine operation in medicine, e.g., in intensive care units.
In order to obtain images that can be well reconstructed and interpreted by means of electroimpedance tomography, a high ratio of the useful signal to the interfering signal is necessary. The frequency spectrum of the background (i.e., the sum of all background signals, which lead to voltages at the electrodes) is not known in case of use of an EIT in any space environment, and, moreover, it is usually subject to change over time. Such interfering signals may be caused, for example, by monitors in the environment (ECG, etc.) or other medical apparatus; when other apparatus are switched on at not too great a distance from the EIT, the electromagnetic interferences, and especially the dominant frequencies of these interferences may change. Thus, the ratio of the useful signal to the background may also change in the course of time at a frequency once set at the beginning.